Antenna systems consisting of an antenna reflector illuminated, directly or indirectly, by one or more RF feed elements are commonly used in equipment for terrestrial and satellite based communications. RF power emitted (or received) by the RF feed is reflected and concentrated by the antenna reflector to (from) a distant receiver (transmitter). The performance of such an antenna system may be specified in terms of the so-called “secondary” pattern characteristics of the antenna reflector, as exhibited, for example, at a remote target. Secondary pattern characteristics may be specified in terms of effective isotropic radiated power (EIRP), gain to noise temperature ratio (G/T), ratio of desired polarization/cross polarization (C/X), far field amplitude, edge of coverage directivity, coverage and suppression (isolation) regions, and cross polarization and co-polarization roll-off.
An important contributor to achieving a specified set of secondary pattern characteristics, are the output characteristics—so-called “primary” pattern characteristics of the RF feed element. Primary pattern characteristics include, for example, aperture efficiency, edge taper, polarization purity, cross polarization level, phase center variation with frequency, and spillover loss. Primary pattern characteristics of the RF feed element may be varied by varying RF feed element design parameters, including, where the RF feed element is a horn, for example, aperture size, length, internal profile, number, size, and placement of horn corrugations, steps, and/or tapers, and size of input waveguide.
Mathematical modeling of the secondary pattern characteristics that result from an antenna reflector being illuminated by an RF feed element having a particular set of primary pattern characteristics is computationally very demanding, at least for antenna systems of commercial interest. As a result, quantitative optimization of RF feed element design parameters with respect to secondary pattern characteristics has proven to be impractical. Instead, antenna system designers conventionally rely on engineering judgment and experience to first specify primary pattern characteristics that are considered necessary to achieve the specified secondary pattern characteristics, and then optimize the RF feed element design parameters with respect to the specified primary pattern characteristics.
For example, referring now to FIG. 1, the conventional design procedure begins with specifying the desired secondary pattern characteristics 101, followed by selection of the reflector optics 102. Selection of reflector optics 102 may include choosing a reflector system configuration, (for example, selecting a single offset reflector or a dual-reflector configuration), along with other choices such as focal length(s), reflector diameter(s), offsets, shaping of reflector surfaces, etc. Specifying primary pattern characteristics 103, may include specifying, for example, phase center variation with frequency, cross polarization level, spillover loss, edge taper. Conventionally, primary pattern characteristics are specified based on engineering judgment that illumination of the reflector system by a feed element compliant with the specified primary pattern characteristics will produce a secondary pattern that at least complies with the specified secondary pattern characteristics. An initial selection 104 is made for the parameters of the feed element, which may include, for example, one or more of: aperture size, length, horn profile, number, size, and placement of corrugations, steps, and/or tapers, size of input waveguide exciting the feed, and possibly others. The design process then enters an iterative optimization loop 105, 106, 107, 108, wherein the primary pattern properties obtained for a given set of feed element parameters are computed and compared to the desired properties. The selected parameters of the feed are adjusted within this loop until a variance (the “first variance”) between achieved and specified primary pattern properties is made sufficiently small. When the first variance is acceptably small, the relatively expensive computation of secondary pattern characteristics may be performed 109. Determination 110 of a variance (the “second variance”) between achieved and desired secondary pattern characteristics is then performed, and a decision 113 is reached as to whether the variance is sufficiently small. If not, the cause must be that the initial choice 103 of primary pattern characteristics was not appropriate, and an adjustment 111 of these characteristics implemented. The feed parameter optimization loop 105, 106, 107, and 108 is then reentered and the process is repeated until convergence on the desired secondary pattern characteristics is achieved.
Results of the foregoing conventional methodology, as measured both in terms of cost and duration of the process, as well as quality of the delivered design, are highly dependent on the quality of the engineering judgment applied to initially specifying the primary pattern characteristics. Expensive redesign and reanalysis and/or sub-optimal antenna system performance are inherent risks of such a design methodology.
Thus, there is a need for more efficient and reliable methods for designing an RF feed element.